Mitochondria are essential organelles found in virtually all eukaryotic cells and show a striking heterogeneity in their number, location and shape in many different cell types. Since the molecules and mechanisms that mediate these processes are largely unknown, we have isolated and analyzed mutants in the yeast Saccharomyces cervisiae defective in mitochondrial fusion and division, in the segregation of mitochondria to daughter cells, and in the maintenance of mitochondrial shape. Our studies have identified several proteins that mediate different aspects of mitochondrial morphogenesis. We are asking the following questions about these new proteins. What is the function of Fzo1p, Ugo1p, and Ugo2p in mitochondrial fusion? fzo1 mutants are defective in mitochondrial fusion and contain numerous mitochondrial fragments. Fzo1p is an outer membrane-localized GTPase. Ugo1p and Ugo2p are new proteins identified in a genetic screen for mitochondrial fusion mutants. How do Dnm1p and Net2p mediate mitochondrial division? dnm1 and net2 mutants are defective in mitochondrial division, and in these cells mitochondria form a single organelle consisting of a network of interconnected tubules. Dnm1p is a dynamin-related GTP-ase located on the mitochondrial surface at sites of mitochondrial division. Net2p is a novel Dnm1p-interacting protein also located on the mitochondrial surface. What is the role of Mmm1p and Slm1p in mitochondrial shape and segregation? mmm1 mutants are defective in the shape of their mitochondria, in the segregation o the mitochondria to daughter cells, and in the maintenance of mitochondrial DNA (mtDNA). We find the Mmm1 protein located in the mitochondrial outer membrane in discrete, punctate structures that also colocalize with mtDNA nucleoids. We speculate that Mmm1p is part of a complex of proteins in the mitochondrial outer and inner membrane that forms a connection between the actin cytoskeleton in the cytosol, and mtDNA in the matrix. slm1 was identified in a screen for mutations that interact with mmm1 mutants. Slm1p, like Mmm1p, is located in the mitochondrial outer membrane and is required for mitochondrial shape. What other proteins are required for mitochondrial dynamics? Recently, a collection of approximately 6000 yeast strains, each disrupted in a different ORF, has been generated. Using a microscope-based assay, we will screen this collection for mutants defective in mitochondria shape, segregation, division or fusion.